diff --git a/dd-java-agent/agent-iast/iast-test-fixtures/src/main/groovy/com/datadog/iast/test/IastRequestContextPreparationTrait.groovy b/dd-java-agent/agent-iast/iast-test-fixtures/src/main/groovy/com/datadog/iast/test/IastRequestContextPreparationTrait.groovy
index 63fe1efac77..22fba9e41b3 100644
--- a/dd-java-agent/agent-iast/iast-test-fixtures/src/main/groovy/com/datadog/iast/test/IastRequestContextPreparationTrait.groovy
+++ b/dd-java-agent/agent-iast/iast-test-fixtures/src/main/groovy/com/datadog/iast/test/IastRequestContextPreparationTrait.groovy
@@ -5,14 +5,19 @@ import com.datadog.iast.IastSystem
 import com.datadog.iast.model.Range
 import com.datadog.iast.taint.TaintedObject
 import com.datadog.iast.taint.TaintedObjects
-import datadog.trace.api.gateway.*
+import datadog.trace.api.gateway.CallbackProvider
+import datadog.trace.api.gateway.EventType
+import datadog.trace.api.gateway.Events
+import datadog.trace.api.gateway.Flow
+import datadog.trace.api.gateway.IGSpanInfo
+import datadog.trace.api.gateway.RequestContext
+import datadog.trace.api.gateway.RequestContextSlot
 import datadog.trace.api.iast.InstrumentationBridge
 import datadog.trace.bootstrap.instrumentation.api.AgentTracer
-import org.slf4j.Logger
-import org.slf4j.LoggerFactory
-
 import java.util.function.BiFunction
 import java.util.function.Supplier
+import org.slf4j.Logger
+import org.slf4j.LoggerFactory
 
 trait IastRequestContextPreparationTrait {
 
@@ -73,7 +78,9 @@ trait IastRequestContextPreparationTrait {
 
       private static Logger withLogger(final String name) {
         final logger = LoggerFactory.getLogger(name)
-        if (logger instanceof ch.qos.logback.classic.Logger) {
+
+        // Check logger class by name to avoid NoClassDefFoundError at runtime for tests without Logback.
+        if (logger.class.name == "ch.qos.logback.classic.Logger") {
           ((ch.qos.logback.classic.Logger) logger).level = ch.qos.logback.classic.Level.DEBUG
         }
         return logger
diff --git a/dd-java-agent/instrumentation/vertx/vertx-web/vertx-web-3.4/build.gradle b/dd-java-agent/instrumentation/vertx/vertx-web/vertx-web-3.4/build.gradle
index 6a0d64a5900..823193ea978 100644
--- a/dd-java-agent/instrumentation/vertx/vertx-web/vertx-web-3.4/build.gradle
+++ b/dd-java-agent/instrumentation/vertx/vertx-web/vertx-web-3.4/build.gradle
@@ -1,4 +1,6 @@
 apply from: "$rootDir/gradle/java.gradle"
+// Use slf4j-simple as default; logback has a high chance of getting stuck in a deadlock on CI.
+apply from: "$rootDir/gradle/slf4j-simple.gradle"
 
 testJvmConstraints {
   // TODO Java 17: This version of vertx-web doesn't support Java 17
@@ -49,3 +51,4 @@ dependencies {
   latestDepTestImplementation group: 'io.vertx', name: 'vertx-web', version: '3.+'
   latestDepTestImplementation group: 'io.vertx', name: 'vertx-web-client', version: '3.+'
 }
+
diff --git a/internal-api/src/main/java/datadog/trace/api/iast/Taintable.java b/internal-api/src/main/java/datadog/trace/api/iast/Taintable.java
index 3b86703d10c..3c221426040 100644
--- a/internal-api/src/main/java/datadog/trace/api/iast/Taintable.java
+++ b/internal-api/src/main/java/datadog/trace/api/iast/Taintable.java
@@ -30,18 +30,23 @@ class DebugLogger {
     private static final Logger LOGGER;
 
     static {
-      try {
-        LOGGER = LoggerFactory.getLogger("Taintable tainted objects");
-        Class<?> levelCls = Class.forName("ch.qos.logback.classic.Level");
-        Method setLevel = LOGGER.getClass().getMethod("setLevel", levelCls);
-        Object debugLevel = levelCls.getField("DEBUG").get(null);
-        setLevel.invoke(LOGGER, debugLevel);
-      } catch (IllegalAccessException
-          | NoSuchFieldException
-          | ClassNotFoundException
-          | NoSuchMethodException
-          | InvocationTargetException e) {
-        throw new RuntimeException(e);
+      LOGGER = LoggerFactory.getLogger("Taintable tainted objects");
+
+      // Check logger class by name to avoid NoClassDefFoundError at runtime
+      // for tests without Logback.
+      if (LOGGER.getClass().getName().equals("ch.qos.logback.classic.Logger")) {
+        try {
+          Class<?> levelCls = Class.forName("ch.qos.logback.classic.Level");
+          Method setLevel = LOGGER.getClass().getMethod("setLevel", levelCls);
+          Object debugLevel = levelCls.getField("DEBUG").get(null);
+          setLevel.invoke(LOGGER, debugLevel);
+        } catch (IllegalAccessException
+            | NoSuchFieldException
+            | ClassNotFoundException
+            | NoSuchMethodException
+            | InvocationTargetException e) {
+          throw new RuntimeException(e);
+        }
       }
     }